services.yml

parameters:

services:
   majax.portal.site_list_builder:
      class: Majax\PortalBundle\Builder\SiteListBuilder
      arguments: [ @doctrine ]
      
   majax.portal.admin.site:
      class: Majax\PortalBundle\Admin\SiteAdmin
      tags:
        - { name: sonata.admin, manager_type: orm, group: sites, label: site }
      arguments: [null, Majax\PortalBundle\Entity\Site, MajaxPortalBundle:SiteAdmin]
      
   majax.portal.admin.user:
      class: Majax\PortalBundle\Admin\UserAdmin
      tags:
        - { name: sonata.admin, manager_type: orm, group: users, label: user }
      arguments: [null, Majax\PortalBundle\Entity\User, MajaxPortalBundle:UserAdmin]
      calls:
        - [ setSiteListBuilder, [ @majax.portal.site_list_builder ] ]

SiteListBuilder.php

<?php
namespace Majax\PortalBundle\Builder;

use Symfony\Bundle\DoctrineBundle\Registry;

class SiteListBuilder {
    /** @var \Symfony\Bundle\DoctrineBundle\Registry */
    private $orm;

    public  function __construct($orm)
    {
        $this->orm = $orm;
    }

    public function getList()
    {
        $repo = $this->orm->getRepository('MajaxPortalBundle:Site');
        $items = $repo->findAll();
        $ret = array();
        foreach($items as $item)
        {
            $ret[$item->getId()] = $item->getName();
        }
        return $ret;
    }
}

UserAdmin.php

<?php
namespace Majax\PortalBundle\Admin;

use Majax\PortalBundle\Builder\SiteListBuilder;
use Sonata\AdminBundle\Admin\Admin;
use Sonata\AdminBundle\Form\FormMapper;
use Sonata\AdminBundle\Datagrid\DatagridMapper;
use Sonata\AdminBundle\Datagrid\ListMapper;
use Sonata\AdminBundle\Show\ShowMapper;

class UserAdmin extends Admin
{
    /** @var \Majax\PortalBundle\Builder\SiteListBuilder */
    private $site_list_builder;

    public function setSiteListBuilder(SiteListBuilder $slb)
    {
        $this->site_list_builder = $slb;
    }

    public function configureShowFields(ShowMapper $showMapper)
    {
        $showMapper
            ->add('username')
            ->add('email')
        ;
    }

    public function configureFormFields(FormMapper $formMapper)
    {
        $formMapper
            ->with('General')
                ->add('username')
                ->add('email')
            ->end()
        ;
    }

    public function configureListFields(ListMapper $listMapper)
    {
        $listMapper
            ->addIdentifier('username')
            ->add('email')
            ->add('lastLogin')
        ;
    }

    public function configureDatagridFilters(DatagridMapper $datagridMapper)
    {
        $datagridMapper
            ->add('username')
            ->add('email')
            ->add('applied_sites', 'callback',
                  array(
                      'filter_field_options' => array('choices' => $this->site_list_builder->getList(), 'expanded' => false, 'multiple' => true),
                      'filter_options' => array(
                          'filter' => array($this, 'handleSitesFilter'),
                          'type' => 'choice',
                          'multiple' => 'true',
                      )
                )
            )
        ;
    }

    public function handleSitesFilter($queryBuilder, $alias, $field, $value)
    {
        if ($value == null)
            return;
        
        $queryBuilder->leftJoin('o.sites', 's');
        $queryBuilder->andWhere($queryBuilder->expr()->in(sprintf('%s.%s', 's', 'id'), $value));
    }

}